1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for timing recovery in digital subscriber line (DSL) communication systems using the discrete multi-tone (DMT) modulation and standard.
2. Discussion of the Related Art
In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, PSTN (public switched telephone network) information, as well as, information from ancillary equipment in analog form (i.e. computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of businesses to reliably transfer information over a broadband network, telecommunications service providers have employed DSL systems to provide a plethora of interactive multi-media digital signals over the same existing twisted-pair copper lines. The provision of asynchronous digital subscriber line (ADSL) systems, using DMT line coding, to customer premises has proliferated over recent years due to the increasing demand for high speed Internet access. ADSL systems can be designed to operate over the same copper lines with either the POTS or a Basic Access Integrated Service Digital Network (ISDN-BA) service.
In a basic DMT implementation, an ADSL transmission unit—central office (ATU-C) is configured to modulate and convert digital data for analog transmission from a central office downstream to a remote location. Each DSL customer is provided an ADSL transmission unit—remote (ATU-R) configured to receive the analog transmission from the telephone line. The ATU-R demodulates the signals and applies error correction before delivering each of the reconstructed digital data signals to its intended device. Concurrently, the ATU-R transmits data from the customer upstream to the central office.
DMT ADSL systems, as defined in the standards, use a transmission bandwidth of up to 1,104 KHz, which is divided into 256 sub-carriers, sometimes referred to as bins or tones. Each of the bins or tones has a center frequency located at an integer multiple of 4.3125 KHz. Generally, certain bins are unused to prevent interference into the POTS or ISDN-BA service, while a larger subset of the bins are used by the downstream transmission. A small subset of the bins is used by the upstream transmission.
The actual transmission spectrum used by a DMT system is adjusted according to the desired data rate based on the transmission characteristics of the particular channel. A DMT communications system initializes by the ATU-C transmitting up to 256 sub-carriers downstream to the ATU-R. The ATU-R measures the quality of each of the sub-carriers and decides whether each particular sub-carrier has a sufficient signal-to-noise ratio (SNR) to be used for service transmission and, if so, how much data each particular sub-carrier should carry relative to the other sub-carriers. After processing the received signals, the ATU-R communicates an optimized bit distribution scheme to the ATU-C via the upstream channel. A bit distribution optimization for upstream transmission is similarly accomplished in the initialization process.
In the ADSL standard, the incoming data streams from each of the possibly separate data services are coded through the use of forward error correcting (FEC) codes, and optionally, the use of trellis coding. The resultant bits parse into groups of bi, i=0, . . . , 255 for each tone at the output of the encoder. The result of the coding is a two-dimensional, or complex, quantity, Xi that consists of a real part and an imaginary part. There are 2bi such complex values for the ith tone, one for each of the 2bi input bit patterns for that tone, dictating its amplitude and phase. The process of encoding repeats for each of the 256 channels 4000 times per second. An inverse discrete Fourier transform (IDFT) of the size 512 (complex to real) converts the complex samples (Xi=X*512-i, i=0, . . . , 255, where * denotes a complex conjugate) into a real sequence of 512 samples xi. The sequence represents the sum of modulated sinusoids. The formula for the IFFT is,                                           x            k                    =                                    1                              512                                      ⁢                                          ∑                                  n                  =                  0                                511                            ⁢                                                X                  n                                ⁢                                  W                  kn                                                                    ,                                  ⁢                              where            ⁢                                                  ⁢            W                    =                                    ⅇ                              j                ⁢                                  π                  256                                                      .                                              Eq        .                                  ⁢                  (          1          )                    Except briefly during initialization, each sample block, or symbol, is transmitted with a cyclic prefix, containing the last 32 samples of the transmit symbol repeated in a prefix that begins each symbol, resulting in 512+32=544 samples per symbol.
Each such prefixed symbol is transmitted 4000 times per second. For reliability, a dummy synchronization symbol of 544 samples is inserted every 69th symbol, so that the aggregate sampling rate at the ATU-C transmitter is (544)(4000)(69/68)=2.208 M samples per second.
The upstream data path is similar, except that a 64-point (128-point optional for ADSL over ISDN-BA) IFFT is used with a prepending of 4 (8 optional for ADSL over ISDN-BA) samples rather than the 512 IFFT with a prepending of 32 symbols used in the downstream path. As a result, the upstream sampling rate in the ATU-R is 276 K samples per second (552 K samples per second for ADSL over ISDN-BA). In order to ensure successful transmission of data in both the downstream and upstream directions, the transmitter and receiver sampling clocks must be synchronized in what is called loop-timing or timing recovery.
When the ATU-R is to provide loop-timing, the goal of timing recovery is to synchronize the ATU-R signal-sampling clock with the ATU-C's sampling clock. Timing recovery is necessary because the exact frequency and phase of the ATU-C is not known beforehand at the ATU-R and because the sampling clock at the ATU-C itself cannot be made exact due to variations in physical devices. After timing recovery has successfully aligned the ATU-C transmitter and ATU-R receiver clocks, synchronization of the upstream signal becomes possible because the ATU-R's analog to digital converter (ADC) and digital to analog converter (DAC) clocks can be synchronized using any of a number of methods known in the art. Alternatively, loop-timing can be provided by the ATU-C instead.
When the frequency (not just the phase) of the ATU-R sampling clock is offset from the frequency of the ATU-C's sampling clock, a number of synchronization mechanisms and phase error generation methods are possible. Two such synchronization mechanisms are pilot timing and “rotors.”
Pilot Timing
In ADSL, the ATU-C transmits a pilot tone at 276 kHz (tone or bin 64 for ADSL over POTS, or bin 96, 414 KHz, for ADSL over ISDN) that is intended to have a constant phase of 45 degrees. Offsets at the output of the ATU-R FFT on this tone from the intended nominal phase of 45 degrees is a phase error that can be used to drive a phase locked-loop to control the sampling clock. The accuracy of the sampling clock must be that any residual phase error noise does not interfere with data transmission. It can be shown that for a signal to noise ratio in excess of 50 dB, the sampling phase deviation should be 500 ps or less. Such a deviation imposes a need for a crystal-based, analog, voltage controlled oscillator if analog phase locking is used.
Rotors
Another form of timing recovery is known as rotors. With rotors, a crystal with a center frequency near the 2.208 MHz sampling rate of ADSL, or a multiple of it, is used to sample the incoming data stream at the ATU-R. The phase error generated from the pilot tone is used to rotate each of the incoming tones by a linearly increasing phase offset determined from an estimate of the timing-offset error. This method assumes that the frequency offset within any symbol period is constant. In practice, this requires a transmit crystal sampling clock with an accuracy of 5 PPM or better. Since a frequency offset will gradually grow over time, a sample will need to be added or deleted from the ATU-R's deletion of the cyclic prefix. While this technique avoids the cost of a highly accurate voltage controlled crystal oscillator, it results in the need for an accurate match of transmit and receive crystals to ensure that the timing offset is within a single symbol of 250 μs. It is significant to note that when the ATU-R is not configured to loop time, that is, synchronize the ADC and DAC clocks, a second pilot tone is necessary to synchronize timing in the upstream direction. ADSL standard tone 16 at 69 kHz has been designated as the upstream pilot tone. A similar timing recovery mechanism can then be implemented in the ATU-C for upstream data, although frequency tolerances are typically relaxed with respect to those in the downstream direction.
Interference to the Pilot Tone Phase
The DMT standards specify that all signals be modulated with the use of inverse discrete Fourier transforms (IDFT). The downstream pilot tone is at bin 64 (276 kHz, ADSL over POTS) and all signal segments shall have this bin overwritten by a 45° complex value of 1+j. The resulting pilot tone signal should have a phase of 45° regardless of the signals appearing in the other bins. This is indeed the case when there is no cyclic prefix. With bin spacing of exactly 4.3125 kHz, a 512-point Fourier transform makes all 256 bins orthogonal to each other. In other words, the SINC function for each bin due to the rectangular time domain windowing is exactly zero at all other bins (see FIGS. 4A and 4B).
However, orthogonality among the bins is destroyed when the 32-point cyclic prefix is introduced. This claim is counter-intuitive because, after all, 32 samples contain exactly 4 cycles of the pilot tone with no phase discontinuities. Nevertheless, the following analysis highlights the problem with pilot tone phase interference.
Consider, for example, the C-REVERB signal in which a fixed pseudo-random symbol is repeated again and again. If the cyclic prefix is introduced, it becomes a part of the repeated sample sequence. A repeated signal sequence has a line spectrum, represented by the Fourier series, which can be found by taking the discrete Fourier transform (DFT) on only one sequence of the samples repeated in the signal. With the cyclic prefix present, the sequence being repeated is 544 samples long, rather than 512 samples, so a 544-point DFT must be applied to find the Fourier series. As a result, the original 256 bins are no longer orthogonal to each other. In other words, the SINC function for each bin is generally nonzero at other bin locations (see FIGS. 5A and 5B). With a line spacing of approximately 4.0588 kHz, the pilot tone comes out in bin 68 and it will have contributions from other bins. Generated according to the DMT standards, the pilot tone in C-REVERB with cyclic prefix will be at 55.91°, or a deviation of +10.91° from the intended 45°. Similarly, the pilot tone in C-SEGUE with cyclic prefix will be at 29.83°, or a deviation of −15.17°. Not surprisingly, C-PILOT is at exactly 45°, with or without cyclic prefix. C-MEDLEY and SHOWTIME signals have random phase interference in each symbol and should average to zero.
It is significant to note that this pilot tone phase interference is not related to the communication channel, since it is generated in the ADSL transmitter itself. No matter how narrow a band-pass filter is used, the phase deviations remain.
Accordingly, it is desired to provide a system and method that efficiently, accurately, and quickly reduces phase interference on the DMT pilot tone at a receiving unit in a DMT communications system.